A method for fabricating a semiconductor device, such as a light emitting device, includes the steps of forming a plurality of semiconductor layers having different conductive types and different carrier concentrations on a semiconductor substrate and forming electrodes on the semiconductor layers. In a method for fabricating a double hereto junction type light emitting device which is most typical of light emitting devices, for example, a three-layer structure consisting of an active layer and a pair of clad layers between which the active layer is sandwiched is formed on a semiconductor substrate. In a method fabricating such a semiconductor device, impurities are introduced into the semiconductor layers by several kinds of methods in order to make the semiconductor layers have desired carrier concentrations.
In one of the methods for introducing an impurity into a semiconductor layer (the first method) for example, an impurity is diffused from an impurity source located on the outside into a semiconductor layer which has been already formed. In the first method, the impurity which is vaporized from the impurity source such as a solid impurity diffusion source is diffused into the semiconductor layer through the surface thereof, and heating treatment of the semiconductor layer is carried out to make the semiconductor layer have a desired carrier concentration.
In another of the methods for introducing an impurity into a semiconductor layer (the second method), an impurity is already added to materials from which a semiconductor layer is generated, and the semiconductor layer having a desired carrier concentration is generated directly from the materials including the impurity. This method is further divided into two classes, one of which is the vapor phase growth method in which a semiconductor layer is grown from vapor phase materials, and the other of which is the liquid phase growth method in which a semiconductor layer is given from liquid phase materials.
However, there are disadvantages in the methods mentioned above. In the first method for introducing an impurity into a semiconductor layer, the impurity is diffused through the surface of the semiconductor layer, so that the carrier concentration profile of the semiconductor layer has a tendency in that the carrier concentration is higher in the vicinity of the surface of the semiconductor layer and becomes lower as the distance from the surface in depth increases.
On the other hand, in the second method for introducing an impurity into a semiconductor layer, the carrier concentration depends on the impurity concentration of the material from which the semiconductor layer is generated or the diffusion temperature, etc., so that it is difficult to adjust the carrier concentration after the semiconductor layer is formed.
In particular, on a double hereto junction type light emitting device, it has been confirmed by the inventors that the light emitting device has a high luminance if a carrier concentration of a clad layer in the vicinity of an active layer is lower than a predetermined value. However, such a specific carrier concentration profile can not be obtained by the methods shown above.